T antigen is apparently the transforming product of SV40. It is also necessary for the initiation of DNA synthesis in infected permissive cells. The biochemical properties of T antigen will be studied: its phosphorylation, cleavage, polypeptide structure, DNA binding and multiple sedimenting forms and possible nucleotide binding in order to ascertain its mechanism of action. The hypothesis that T antigen is a self-associating protein which binds cooperatively to the double-stranded SV40 DNA as isologous dimers or tetramers at the replication initiation site will be tested. The hypothesis that T antigens from SV40-infected and transformed cells are functionally different will also be tested. The methods to be used will be immunoprecipitation, electrophoresis, tryptic mapping, and autoradiography of radiolabeled T antigen as previously employed. I intend to work out methods for producing antibody to gel-purified T antigen, and for biochemical purification of T antigen in microgram quantities.